1. Field of the Invention
This invention relates to a method for grafting a preformed hydrophilic polymer such as polyvinyl alcohol onto the surface of hydrophobic organic polymer substrates. This invention also relates to coated organic polymer substrates prepared using this method.
2. Background Information
It is known to apply coatings of various materials onto the surface of substrates formed from hydrophobic polymers. Coatings of liquid or gel type lubricating materials are particularly desirable for tubing and catheters that are inserted into blood vessels and various body cavities in animals and humans, for medical devices such as surgical gloves, sutures, contact lenses, cardiac pacemakers and associated leads, artificial implants, and peristaltic pump chambers.
One type of lubricating coating is temporary, and includes non-reactive oils such as vegetable oils and silicone oils (typically low molecular weight polydiorganosiloxanes). The disadvantage of these coatings include their temporary nature and the tendency of the coatings to rub off and stain materials they come in contact with.
Methods for applying a more permanent hydrophilic coatings to hydrophobic substrates are also known. Japanese Patent Publication (Kokai) no. 60/171140 describes filling the pores of a polymerized perfluorohydrocarbon polymer with a water-soluble monomer such as 2-hydroxyethyl methacrylate that is subsequently polymerized and crosslinked in situ. Treatment of a polyurethane substrate with a salt to produce a porous structure, filling the pores with gelatin and crosslinking the gelatin using glutaraldehyde is taught in an article by L. Rhode that appeared in Wiss. Z. Wilhelm-Pieck-Univ. Rostock, Naturwiss Reihe, 33 (7), 14-18, 1984.
A method for increasing the durability of hydrophilic coatings using an interpenetrating polymer network is described by M. Dror et al. in Biomater., Med. Devices, Artif. Organs, 7(1), 31-9, 1979. The network is formed by dissolving a crosslinkable polymer in a solvent capable of swelling the substrate, exposing the substrate to the resultant solution and then crosslinking the polymer when it has been absorbed into the swollen substrate. A major disadvantage of this technique is that it requires use of organic solvents, which typically adversely alters the physical properties of the substrate in addition to the recovery and/or disposal problems associated with the use of organic solvents.
Hydrophilic coatings can be covalently bonded to hydrophobic substrates if the inert surface of the substrate is first activated by exposure to a hydrogen plasma or high energy radiation and then reacted with at least one water soluble monomer. As an example of this technique, Japanese patent publication 75/38790 published in 1975, teaches exposing a polyvinyl chloride substrate to ionizing radiation in the presence of a mixture of oxygen and ozone to form oxides and peroxides that are used as active sites for the graft polymerization of 2-hydroxyethyl methacrylate.
The use of radiation from a .sup.60 Co source to initiate the graft polymerization of a water soluble monomer such as 2-hydroxyethyl methacrylate, N-vinyl pyrrolidinone and ethylene glycol dimethacrylate on to a silicone rubber substrate is described in German patent 2,515,671, which issued in 1975.
B. Rattner in published German patent application No. 2,515,671 teaches using radiation to graft polymerize various acrylic monomers onto the surface of silicone rubber in the presence of cupric nitrate.
The disadvantages of using high energy radiation to initiate graft polymerization of a monomer onto a substrate are the cost of the equipment required to generate the radiation. The radiation may also cause sub-surface damage to the substrate, thereby adversely affecting its physical properties.
One way to avoid the use of high energy radiation is to use a coupling agent containing groups that form covalent or other types of bonds with a subsequently formed hydrophilic polymer. S.C. Lin et al. in Adv. Biomater., (Clin. Appl. Biomater) 4, 245-52 (1985) teach treating a silicone rubber substrate with vinyltriacetoxysilane which was then copolymerized with N-vinyl pyrrolidone to form a coating that was not only adsorbed onto the surface of the substrate but also absorbed into it.
The use of isocyanate groups to bond hydrophilic polymers to hydrophobic substrates is disclosed in U.S. Pat. No. 4,100,309, which issued on Aug. 29, 1978, and U.S. Pat. No. 4,876,126, which issued to Takemura et al. on Oct. 24, 1989. The high reactivity and toxicity of the isocyanate group make its presence undesirable in medical devices and equipment.
PCT International Patent Application WO84/00908 teaches using a solubilized 2-hydroxyethyl methacrylate/methacrylic acid copolymer and an unvulcanized surgical glove that had previously been primed using aluminum sulfate, and then vulcanizing the glove.
Graft polymerization on to hydrophobic substrates of various monomers forming hydrophilic polymers is also described in the following references:
Seifert et al. in the Journal of Biomaterials Research, 19, 9, 1043-71 (1985) using copolymers of 2-hydroxyethyl methacrylate and N-vinylpyrrolidone; PA0 Bamford et al. in Eur. Poly. Journal, 19, 1027-35 (1983) using a hydrogel-forming monomer in the presence of the combination of an N-bromo- or N-chloroamide or -imide and a transition metal carbonyl; PA0 Chapiro in Eur. Poly. Journal, 19, 859-61 (1983) using radiation-induced grafting of N-vinylpyrrolidone; PA0 Jendrychouska-Bonamour in J. Poly. Sci, Polym. Chem. Ed., using radiation-induced grafting of acrylic acid onto a poly(methyl methacrylate) substrate, followed by reaction of the resultant copolymer with carbodiimide or glutaraldehyde; PA0 Beach et al. in a US Government report (NIH -N01-HV-4-2950-6) using .sup.60 Co radiation to graft the sodium salt of a crotonic acid/vinylacetate copolymer from a solution of the corresponding monomers; and PA0 J. Kearney et al. in the Am. Chem. Soc., Div. Org. Coat. Plast. Chem. , Pap. , 33 (2), 346-50, (1973) using an aqueous solution of acrylamide, a crosslinking agent and a transition metal salt as the polymerization catalyst to prevent homopolymerization in the solution.
It is known that ceric salts, particularly double salts containing amino and/or nitro groups, will initiate polymerization of vinyl monomers such as the esters monomers of acrylic and methacrylic acids. This reaction is believed to involve formation of peroxides and was first investigated by G. Mino and S. Kaizerman reporting in the Journal of Polymer Science, 31, 242-3 (1958). The use of a ceric salt as an initiator to graft polymerize acrylamide onto pulp is described in German patent no. 2,416,531, which issued during 1974.
Amudeswari et al. in Curr. Sci. 52, 2, 58-9 (1983) describe the graft polymerization 2-hydroxyethyl methacrylate on to a powdered collagen using a cerium compound as the initiator.
Xin et al in Makromol. Chemie, 186, 8, 1533-41 (1985) used acrylamide or the combination of tetramethylene glycol and a polyfunctional carboxylic acid as the monomer(s) and a polyurethane as the substrate. The monomer was polymerized using a ceric salt as the polymerization initiator.
The conditions of the graft polymerization reactions described in the foregoing references using ceric compounds as the polymerization initiator result in substantial amounts of homopolymerization in addition to crosslinking and grafting of the polymer onto the surface of the substrate. In the absence of a means to control the homopolymerization, the properties of the final coating could vary substantially, depending upon relative rates of the competing polymerization and grafting reactions.
The homopolymers not grafted to the substrate together with unreacted monomers typically must be removed from the coated substrate, particularly if the final product is intended for use as a medical device.
The preparation substrates coated with hydrophilic polymers and control of the properties exhibited by these substrates would be considerably easier if it were possible to graft a preformed polymer to the surface of substrate rather than trying to reproducibly balance the competing homopolymerization and grafting reactions.
The present inventors discovered that this desirable goal can be achieved by contacting a hydrophobic organic polymer containing polar groups such as hydroxyl, carboxylate and urethane, or a polyorganosiloxane with an aqueous solution of certain hydrophilic organic polymers in the presence of certain ceric compounds.